1. Field of the Invention
This invention relates to electrical junction devices that are subject to a p dopant diffusion from a heavily p doped region to a more lightly n doped region, and to structures for inhibiting such diffusion.
2. Description of the Related Art
A heterojunction bipolar transistor (HBT) is an example of an electrical junction device that has a highly p doped region from which dopant can diffuse into a nearby more lightly doped n region. Other devices that have such p-n junctions include zener diodes, tunneling diodes, and lasers with an interface between a heavily p doped cladding layer and a more lightly n doped or even intrinsic active region. However, since the development of the present invention has occurred in connection with HBTs, in which a diffusion of a p base dopant to the n doped emitter region is a particular problem, the following discussion will be in terms of this type of device.
HBTs represent a field of increasing interest because of their potential for higher emitter efficiency, decreased base resistance, less emitter current crowding, improved frequency response and wider temperature range of operation. A representative HBT is illustrated in FIG. 1. It includes a semi-insulating semiconductor substrate 2, typically InP or GaAs. A highly doped n.sup.+ subcollector layer 4 on the substrate provides an underlying contact with the more lightly n.sup.- doped collector layer 6, with a metallized or highly doped semiconductor pad 8 providing a collector contact through the subcollector. A p.sup.+ base layer 10 is formed over the collector, with an n doped emitter 12 over the central portion of the base, and a lateral base contact 14 surrounding the emitter. An emitter contact 16 is provided on the emitter's upper surface.
During the operation of the HBT, the input voltage forward-biases the base-emitter junction, allowing electrons from the n-type emitter to enter the base. The injected electrons travel across the base by diffusion or drift, and are swept into the collector when they reach the base-collector junction by the high fields in this region. In a homojunction bipolar transistor there is a flow of holes from the base into the emitter across the forward biased base-emitter junction. To suppress the reverse hole injection, the base doping is made lower than that of the emitter. This, however, makes the base layer more resistive, thereby increasing the device's overall base resistance and degrading its bandwidth. In an HBT, by contrast, a wide-bandgap emitter material is used that creates a higher energy barrier for hole injection into the emitter than the barrier to electron flow into the base, thereby automatically suppressing the base current that is due to hole injection. The doping of the base layer can be made as large as possible from solid state chemistry considerations, and the base resistance is correspondingly reduced.
To achieve the high base dopings necessary for RF performance, typically 5-10.times.10.sup.19 cm.sup.-3 beryllium is widely used as the p type base dopant for npn HBTs. A significant reliability issue with such devices is that the electric currents and fields generated at the base-emitter junction cause Be to diffuse from the base into the emitter. This movement of Be is characterized by an increase in the emitter-base turn-on voltage as the electrical junction (defined as the location where the dopant concentration of Be equals the n dopant concentration of the emitter) is pushed further into the emitter. The Be penetration leads to a reduction in gain, and eventually to complete failure of the transistor. This degradation mechanism is currently the operating rate limiting failure mechanism of npn AlInAs/GaInAs HBTs.
The large variability in turn-on voltage is a result of this voltage's dependence upon the bandgap energy at the electrical junction location. This can be expressed as: EQU V.sub.eb .alpha.Eg/q+Kln(Ic),
where V.sub.eb is the emitter-base turn-on voltage, Eg is the bandgap energy at the electrical junction, q is the electron charge, K is a constant, in is the natural logarithm function and Ic is the HBT's collector current. The position of the electrical junction is established at the point where the n-dopant concentration in the wide bandgap emitter (typically silicon) equals the p-type Be dopant concentration in the narrow bandgap base.
For the example of an AlInAs/GaInAs emitter/base junction, the bandgap energy of AlInAs is 1.45 ev, while that of GaInAs is 0.75 ev. If the electrical junction moves from the more heavily p-doped GaInAs base into the n-doped AlInAs emitter due to Be diffusion, the device's turn-on voltage will increase in proportion to the difference between the bandgap energies of the emitter and base materials, which in this case would be about 700 mV. For some applications such as analog-to-digital converters (ADCs), by contrast, it is desirable for the emitter-base turn-on voltage to be controlled to a level of a few mV. For an abrupt emitter-base junction, a movement of the electrical junction by only tens of Angstroms can shift the junction position out of the base and into the emitter, resulting in significant turn-on voltage changes on the order of hundreds of mV.
In a typical HBT, the base doping concentration is 1-2 orders of magnitude higher than the emitter doping concentration. Be is typically used as the base dopant. It diffuses rapidly, with a concentration-dependent diffusion constant that increases at higher doping levels and can exhibit "explosive" degrees of diffusion when a sufficiently high level of doping (greater than 10.sup.19 cm.sup.-3) is attempted. Therefore, the electrical junction is typically determined by Be diffusion into the emitter. This results in a very nonreproducible junction position from wafer to wafer, and even at different locations on a single wafer, since the Be diffusion from the heavily doped base is very sensitive to wafer temperature during growth. In addition, if different base dopings are required, the different Be doping levels will diffuse to different degrees, and this in turn will result in different positionings of the electrical junction.
One approach to solving the Be diffusion problem uses an undoped layer of low bandgap material, such as GaInAs or GaAs about 100-500 Angstroms thick, between the wide band-gap emitter and the low bandgap base. The undoped layer acts as a buffer region into which the base Be can diffuse, so that the bulk of the diffusing Be does not reach the emitter. This technique is described in Malik et al., "High-gain, high frequency AlGaAs/GaAs graded band-gap base bipolar transistors with a Be diffusion set back layer in the base" Applied Physics Letters, Vol. 46, No. 6, pages 600-602, (1985), and in Jalali et al., "New ideal lateral scaling in abrupt AlInAs.InGaAs heterostructure bipolar transistors prepared by molecular beam epitaxy" Applied Physics Letters, Vol. 54, No. 23, pages 2333-2335, (1989). Undoped setback or spacer layers of base material between the base and emitter are also disclosed in Jensen et al., "AlInAs/GaInAs HBT Ic Technology", IEEE Journal of Solid-State Circuits, Vol. 26, No. 3, March 1991, pages 415-421, and in Nottenburg et al., "In P-Based Heterostructure Bipolar Transistors", Proc of the 1989 GaAs IC Symposium, 1989, pages 135-138.
When using the setback approach, some of the Be must still diffuse into the wide bandgap emitter, or the resulting undoped region between the emitter and base can result in gain degradation and increased base resistance. This approach does reduce the actual amount of Be that diffuses into the emitter, but the positioning of the emitter-base electrical junction is still dependent upon how far the Be diffuses into the emitter, and control of the emitter-base turn-on voltage is quite poor. An additional complication is that different base doping levels will result in different degrees of Be penetration into the emitter.
Another approach has been to replace the abrupt heterointerface at the emitter-base junction with a region in which the alloy is graded between the emitter and base materials. In Won et al, "Self-Aligned In.sub.0.52 Al.sub.0.48 As/In.sub.0.53 Ga.sub.0.47 As Heterojunction Bipolar Transistors with Graded Interface on Semi-Insulating InP Grown by Molecular Beam Epitaxy", IEEE Electron Device Letters, Vol. 10, No. 3, March 1989, pages 138-140, a graded interface is provided between the base and emitter that makes a linear and continuous transition from the base to the emitter material. In Asbeck et al., "InP-based Heterojunction Bipolar Transistors: Performance Status and Circuit Applications", Second Int'l Conf. on Indium Phosphide and Related Materials, Apr. 23-25, 1990, pages 2-5, a short period superlattice is formed between an AlInAs emitter and a GaInAs base. The superlattice consists of four sections, with each section having four identical periods; each period had a thickness of 12 Angstroms. The ratio of the width of emitter material to the width of base material in each superlattice period varied from about 4:1 in the section adjacent the emitter, to about 1:4 in the section adjacent the base. This structure was proposed as an alternate to a continuously graded interface because of the latter's fabrication difficulties.
Continuously graded junctions of the Won et al. type have been used to avoid conduction spikes at the emitter-base junction, and to thus enhance charge transport and current gain, but they do not solve the Be diffusion problem. The advantage of the approach is that the bandgap slowly changes over several hundred Angstroms, instead of abruptly changing over tens of Angstroms. Since the change in V.sub.eb is proportional to the change in bandgap, for a given amount of diffusion V.sub.eb will change much less for a graded than for an abrupt junction. However, even though the Won et al. method lessens the sensitivity of V.sub.eb to Be diffusion, it is still the diffusion of the Be from the heavily doped base that determines the position of the emitter-base electrical junction. In the Asbeck et al. superlattice structure, no reduction in field-enhanced diffusion of Be from the base was noted.
Graded superlattices between p and n doped regions have previously been used in contexts other than HBTs. A graded superlattice was employed to eliminate the interface pile-up effect of holes in a "high-low" InP/GaInAs avalanche photodiode in Capasso et al., "Psuedo-quaternary GaInAsP semiconductors: A new Ga.sub.0.47 In.sub.0.53 As/InP graded gap superlattice and its applications to avalanche photodiodes", Applied Physics Letters, Vol. 45, No. 11, Dec. 1, 1984, pages 1193-1195, and to minimize the effect of carrier trapping at the AlInAs/GaInAs interface of a metal-semi-conductor-metal photodiode in Wada et al., "Very high speed GaInAs metal-semiconductor-metal photodiode incorporating an AlInAs/GaInAs graded superlattice", Applied Physics Letters, Vol. 54, No. 1, Jan. 2, 1989, pages 16-17. Neither of these structure purported to block a diffusion of p dopant into an n doped region.